Conventional three dimensional printing processes take various forms. Nearly all involve the slicing of a software representation of a three-dimensional article into software representations of two-dimensional slices of the article and then building the article in three-dimensions by sequentially transforming such two-dimensional representations into physical layers built one upon another. Several three-dimensional printing processes make use of particulates (also sometimes referred to in the art as “powder” or “particles”) for building the article in three dimensions. Among these processes are the binder jetting process (also known as the inkjet printing process), selective layer sintering process, selective laser melting process, direct metal laser sintering process, electron beam melting process, and the selective heat sintering process.
In the three-dimension printing processes that use particulates, a first layer of the particulates is deposited onto the top surface of a build platform. This deposition is sometimes referred to in the art as “spreading” a particulate layer. An image of the two-dimensional representation of the first slice of the article may then be imparted to this first particulate layer or the first particulate layer may be covered over with one or more additional particulate layers before the image of the first slice of the article is imparted to the then-topmost particulate layer. After that, the sequence of applying a particulate layer and imparting the image of the two-dimensional representation of a subsequent slice of the article is performed until the three-dimensional article is formed. The top surface of the bare build platform or of the then-topmost particulate layer is referred to herein as the “build surface”. Often, more than one article or multiple copies of the same article are produced at the same time by simultaneously imparting the respective two-dimensional slices of the articles onto the build surface particulate layers. At the end of the particulate layer-placing plus image-imparting iterative sequence, particulate-based versions of the article or articles are surrounded by a bed of the unbonded particulates. This bed is sometimes referred to as a “build bed” or as a “powder bed” or as a “particulate bed”.
The particulate processes commonly use a support platform which is designed to be step-wise lowered into a walled cavity. At the start of the process, the support platform is positioned so that the support surface is flush with the top of the cavity walls. After each particulate layer-placing plus image-imparting iteration, the support platform is indexed down into the cavity so that the then-topmost particulate layer is flush with the cavity walls so that the next particulate layer can be deposited.
Various techniques have been devised for depositing the particulate layers, but a common problem occurs with particulate layer deposition that is due to the nature of particulate flow in normal-level gravity fields. Unlike layers of continuous solid materials, e.g. sheets of metal, plastic, or paper, the particulate layers do not terminate in sharply-defined vertical walls, but rather in somewhat irregular edges with generally downwardly-outward sloping walls, the contours of which roughly relate to the angle of repose of the particulates and depend on various material and dynamic factors, e. g. the inter-particle attractive/repulsive forces of the particulates, the velocity vectors active on the particulates during layer deposition, the presence of additives, coatings, or absorbed or adsorbed chemical species on the particulates, environmental forces such as vibrations, etc. Usually, in order to assure that the build surface always has the same predictable dimensions of the initial support surface, an excess amount of particulates is deposited for each layer. However, unless some provision is made for removing the excess deposited particulates, it is likely that the accumulation of the excess deposited particulates after the deposition of one or more layers will interfere with the desired deposition of additional layers.
A common way of handling the excess deposited particulate problem is to provide one or more receiving troughs into which the excess deposited particulate can fall or be pushed. However, sizing the receiving troughs can be problematic. Making the troughs too large requires making the overall size of the three-dimensional printing apparatus larger than it needs to be. Making the troughs too small may result in the troughs becoming ineffective upon overfilling or require the use of reservoirs which take in the particulates directly or indirectly from the trough or troughs. Also, adding to the problem is the fact that the effective bulk density of the particulate as deposited can change from one type of particulate to another and even from batch-to-batch for the same type of particulate. Furthermore, in order to prevent cross-contamination, the troughs and associated reservoirs must be thoroughly cleaned before the three dimensional printing apparatus can be utilized with another type of particulate.